Pressure reservoir for exerting pressure on a hydraulic system, with which preferably a gas exchange valve of an internal combustion engine is actuated

ABSTRACT

A pressure reservoir is used to exert pressure on a hydraulic system with which, a gas exchange valve, for instance, of an internal combustion engine can be actuated. The pressure reservoir includes a housing and a piston that is prestressed in operation by a device. To enable making the pressure reservoir as small as possible, it is proposed that the device which prestresses the piston of the pressure reservoir has a characteristic force-travel curve, in one range of motion of the piston, that has a slope which differs from the slope in a different range of motion of the piston.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 USC 371 application of PCT/DE 02/00079, filedon Jan. 12, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pressure reservoir for exertingpressure on a hydraulic system, with which preferably a gas exchangevalve of an internal combustion engine is actuated, having a housing anda piston prestressed in operation by a device.

2. Description of the Prior Art

A hydraulic system with a pressure reservoir of the type with which thisinvention is concerned is known from German Patent Disclosure DE 193 26047 A1. A hydraulic system of this kind is used for instance foractuating the inlet and outlet valves of an internal combustion engine,if the engine does not have a camshaft. Such an engine has the advantagethat the control times of the inlet and outlet valves are independent ofthe position of the piston of the applicable cylinder. Depending on theengine operating state, such as high rpm, and on the torque desired bythe driver, valve opening and closing times can be achieved which makeespecially optimal engine operation possible in terms of emissions andfuel consumption.

The known hydraulic system functions with a hydraulic circuit, which issupplied from a hydraulic reservoir via a high-pressure hydraulic pump.An actuating device includes a piston that can be acted uponhydraulically in both directions of motion and that is connected to thevalve shaft of a gas exchange valve, such as an inlet valve. Via 2/2-wayvalves, one at a time of the two chambers of the hydraulic cylinder canbe subjected to higher pressure, which leads to a corresponding motionof the piston and as a result to an opening or closing event of the gasexchange valve of the engine block.

The hydraulic circuit communicates with a hydraulic pressure reservoir,which is embodied as a spring-loaded piston reservoir and serves to dampvibration in the hydraulic system. An identically embodied emergencypressure reservoir also communicates with one of the two chambers in thehydraulic cylinder; if the pressure drops in the hydraulic line, thisemergency pressure reservoir still furnishes sufficient pressure and asufficient fluid volume to enable the gas exchange valve to be moved toits closed position of repose. The two pressure reservoirs operate atdifferent pressure levels, which are set by means of differentstiffnesses of the corresponding restoring springs. From DE 198 26 047A1, it is also known to use only a single pressure reservoir, whichfunctions simultaneously as both a working pressure reservoir and anemergency pressure reservoir.

If only a single pressure reservoir is provided, its design must be suchthat at minimal operating pressure in the hydraulic system, sufficienthydraulic medium is stored to enable reliably moving the gas exchangevalve into the closed position of repose in the event of an emergency.This requires a relatively soft spring and a long spring travel. Inorder at the same time to assure that over the entire operating pressurerange, a sufficient damping action exists, this kind of pressurereservoir, equipped with a soft spring, must be very long structurally,as a function of the minimum and maximum operating pressure. Such alarge pressure reservoir, however, can be accommodated only withdifficulty in the available installation space in an internal combustionengine. Moreover, because of the great structural length, in theoperating pressure range a relatively large volume of fluid must bestored in such a pressure reservoir, and as an idle volume, beyond thedesired damping action, this adversely affects the dynamics of thehydraulic system.

It is therefore the object of the present invention to refine a pressurereservoir of the type defined at the outset such that on the one hand, apressure damping function and on the other an emergency pressurefunction are available, while nevertheless the pressure reservoir is assmall as possible.

The above and other objects and advantages are attained, in a pressurereservoir of the type defined at the outset, by providing that thedevice which prestresses the piston of the pressure reservoir has acharacteristic force-travel curve, in one range of motion of the piston,that has a slope which differs from the slope in a different range ofmotion of the piston.

According to the invention, a prestressing device with a nonlinearcharacteristic is used in the pressure reservoir. It is understood thenthat first, when the piston is urged out of its pressureless position ofrepose, a softer characteristic of the prestressing device is desired;that is, a change in pressure results in a relatively long movementdistance of the piston. In a range of motion of the piston that is faraway from the position of repose of the piston, conversely, a stiffercharacteristic of the prestressing device of the piston is desired; thatis, a pressure change should cause only a comparatively slight motion ofthe piston.

In this way, both desired functions, namely the emergency pressurefunction and the vibration damping function, can be achieved in a singlepressure reservoir: The emergency pressure function is available in therange of motion of the piston of the pressure reservoir in which theprestressing device has a relatively soft characteristic. Within thispiston range of motion, the pressure reservoir is thus already capable,at only a slight pressure drop, of dispensing a large enough fluidvolume into the hydraulic circuit for securing, for instance a gasexchange valve, in the event of a pressure loss. The vibration dampingfunction exists in the range of motion of the piston within which thecharacteristic force-travel curve is comparatively steep. In this pistonrange of motion, even major pressure fluctuations result in only aslight piston motion. Accordingly, in this piston range of motion, it isalso possible for only a slight movement distance of the prestressingdevice to be provided, which in turn is favorable for the sake of ashort structural length of the pressure reservoir.

The pressure reservoir of the invention can accordingly be used on theone hand for storing a fluid volume for emergency operation, and on theother, it can be used in normal operation for vibration damping, and atthe same time is very small in size. It can therefore be integratedeasily and without problems into the available installation space.Furthermore, because of the slight fluid volume stored and the greatstiffness of the prestressing device, an optimal vibration damping canbe achieved in normal operation without impairing the system dynamics.

In a first refinement, the device which prestresses the piston of thepressure reservoir has at least two series-connected devices, withcharacteristic force-travel curves of different slope, which prestressthe piston in operation. The desired properties of such a pressurereservoir can be achieved especially easily, since in it, the variousfunctions are also performed physically separately.

It is especially preferred that the devices for prestressing the pistoninclude at least two series-connected springs, and the stiffness of onespring differs from that of the other spring. A pressure reservoir withthis kind of two-stage spring assembly can be constructed simply andvery economically and furthermore is robust.

In an especially preferred feature of the pressure reservoir of theinvention, the pressure reservoir has an elongated part with two endportions and one support portion, which is disposed between the endportions and has a larger outer dimension than the end portions and onwhich two adjacent springs are braced, the one spring being tightened inoperation between one side of the support portion and the piston, andthe other spring being tightened between the other side of the supportportion and a housing portion. An elongated part of this kind enablesthe secure guidance of the piston, on the one hand, and of thecorresponding springs, on the other.

It is also provided that at least two stops are provided, which preventthe springs from being tightened into a block in operation. Essentially,tightening springs into a block has two disadvantages: First, mostsprings, in the range of motion located just before tightening into ablock occurs, exhibit a markedly nonlinear, and above all oftennon-replicable, characteristic curve behavior. This is unwanted in thepresent case as well. Furthermore, whenever the springs are tightenedinto a block, wear of the touching surfaces of the springs can occur,which can impair the service life of the springs. The stops according tothe invention prevent this.

Especially simply, such stops can be realized in conjunction with theabove-described elongated part: In this case, the length of theelongated part can be adapted such that one axial end of the elongatedpart forms a stop with a housing portion of the pressure reservoir, andthe other axial end of the elongated part forms a stop with the piston.

Basically, all types of springs are suitable for the pressure reservoirof the invention. Examples are spiral springs, air springs and magnetsprings. It is especially preferred, however, that at least one of thesprings is a cup spring. The use of cup springs, because of the betterratio between the spring work and the installation space, brings about afurther reduction in the structural length of the pressure reservoir.Moreover, because of the strong friction damping in a cup springassembly, the damping action of the reservoir is enhanced.

The invention also relates to a hydraulic system for actuating a gasexchange valve of an internal combustion engine, in particular of amotor vehicle, having a fluid reservoir, a fluid pump, a fluid line, apressure reservoir that communicates with the fluid line having ahousing and a piston prestressed in operation by a device, and having anactuating device, which communicates via a valve device with the fluidline and actuates the gas exchange valve.

To reduce the overall dimensions of the hydraulic system, it is proposedthat the pressure reservoir be embodied as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, exemplary embodiments of the invention are described in detail,in conjunction with the accompanying drawings, in which:

FIG. 1, a basic illustration of a hydraulic system for actuating a gasexchange valve of an internal combustion engine;

FIG. 2, a section through a first exemplary embodiment of a pressurereservoir of the hydraulic system of FIG. 1;

FIG. 3, a pressure and travel graph to explain the function of thepressure reservoir of FIG. 2;

FIG. 4, a schematic section through a second exemplary embodiment of apressure reservoir;

FIG. 5, a schematic section through a third exemplary embodiment of apressure reservoir;

FIG. 6, a schematic section through a fourth exemplary embodiment of apressure reservoir; and

FIG. 7, a schematic section through a fifth exemplary embodiment of apressure reservoir.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a hydraulic system referred to overall by reference numeral10 serves to actuate a gas exchange valve, which here is embodied as aninlet valve 12 of an internal combustion engine 14.

The inlet valve 12 is actuated by a hydraulic cylinder 16. This cylinderincludes a housing 18, in which a piston 20 with a piston rod 22 isguided slidingly. The piston rod 22 is passed through the housing 18 andis connected to a valve shaft 24, which in turn is formed onto aplatelike valve element 26. In the closed state of the inlet valve 12,the valve element 26 rests tightly against a valve seat 28 in the upperregion of a combustion chamber 30 of the engine 14. If no hydraulicpressure is available, the piston 20 is pressed upward by a spring 32,and as a result the inlet valve 12 is closed.

The hydraulic system 10 further includes a supply container 34, fromwhich hydraulic fluid is pumped by a high-pressure pump 36 into ahigh-pressure hydraulic line 38. Downstream of a check valve 40, thehigh-pressure hydraulic line 38 branches off into one branch 42, whichdischarges directly into a lower work chamber 44 of the hydrauliccylinder 16. Another branch 46 of the high-pressure hydraulic line 38leads to a 2/2-way switching valve 48, which in the currentless state ispressed into its closed position by a spring 50. The branch 46 of thehigh-pressure hydraulic line 38 leads, downstream of the 2/2-wayswitching valve 48, to an upper work chamber 52 of the hydrauliccylinder 16. From there, a high-pressure hydraulic line leads, via afurther 2/2-way switching valve 56 and a check valve 58, back to thesupply container 34. The 2/2-way switching valve 56 is opened by aspring 57, in the currentless state.

A tie line 60, which communicates with a pressure reservoir 62,discharges at the point where the high-pressure hydraulic line 38branches off into the branch 42 and the branch 46. The construction ofthe pressure reservoir is shown in detail in FIG. 2.

The pressure reservoir 62 includes a housing 64, which has an overallcylindrical shape, and in which a cylindrical hollow chamber 66 isembodied. On the right-hand side, in FIG. 2, the hollow chamber 66 isclosed with a cap 68, while conversely, on the left-hand side in FIG. 2,it communicates with the tie line 60 via a connecting conduit 70. Thecap 68 has a valve opening, which in the present exemplary embodiment islocated outside the sectional plane and is therefore not visible.

A piston 72 is retained displaceably in the hollow chamber 66. Theradial jacket face of the piston 72 is sealed off from the inner wall ofthe hollow chamber 66 by a sealing ring 74, which is placed in anannular groove 76 in the outer jacket face of the piston 72. A pistonrod 78 is formed onto the piston 72. It extends from the piston 72toward the cap 68. The piston 72 and the piston rod 78 are coaxial tothe hollow chamber 66 of the housing 64 of the pressure reservoir 62.

Coaxially to the piston 72 and to the piston rod 78, there is anelongated tubular part 80 located in the hollow chamber 66 of thepressure reservoir 62. The elongated tubular part 80 is slipped onto thepiston rod 78 in sliding communication. The elongated tubular part 80includes a cylindrical end portion 82, located on its left-hand side interms of FIG. 2, and a cylindrical end portion 84, located on itsright-hand side in FIG. 2. Located between the two end portions 82 and84 is a support portion 86, whose outside diameter is greater than theoutside diameter of the left-hand end portion 82 and of the right-handend portion 84. In other words, the support portion 86 takes the form ofan annular collar.

Between the support portion 86 and the piston 72, a packet 87 of a totalof twelve cup springs 88 (for the sake of simplicity, not all the cupsprings 88 have reference numerals in the drawing) is disposed coaxiallyto the piston 72, piston rod 78, and elongated tubular part 80. Thepacket 87 is divided into four individual groups (not carrying referencenumerals), each comprising three parallel cup springs 88. A packet 89comprising three parallel cup springs 90 is disposed between the supportportion 86 and the cap 68 of the housing 64.

In the pressureless state of repose, shown in FIG. 2, of the pressurereservoir 62, the cup springs 88 and 90 are relaxed. In this state,there is a free space between the axial end, on the left in FIG. 2, ofthe elongated tubular part 80 and the piston 72. A free space is alsopresent between the right-hand axial end, in the drawing, of theelongated tubular part 80 and the bottom of a recess 92 in the cap 68 ofthe housing 64. The cup springs 88 are all softer than the cup springs90. The spring travel of the packet formed of the cup springs 88 isoverall longer than the spring travel of the group formed by the cupsprings 90.

The hydraulic system 10 shown in FIG. 1, having the pressure reservoir62 shown in FIG. 2, functions as follows:

The high-pressure pump 36 pumps hydraulic fluid out of the supplycontainer 34 into the hydraulic line 38 and from there via the branchline 42 into the lower work chamber 44 of the hydraulic cylinder 16.When the switching valve 48 is opened and the switching valve 56 isclosed, the upper work chamber 52 of the hydraulic cylinder 16 is alsoput under pressure by hydraulic fluid. Since the engagement area in theaxial direction on the top side of the piston 20 of the hydrauliccylinder 16 is greater than on its underside, the piston 20 is presseddownward in this case, and the inlet valve 12 is opened.

If the switching valve 48 is closed and the switching valve 56 isopened, the upper work chamber 52 is made to communicate, via the branchline 54, with the ambient pressure, and as a result the piston 20 ismoved upward again, and the inlet valve 12 is closed. In this way,without having to trigger the inlet valve 12 mechanically, for instanceby means of a camshaft of the engine 14, very fast opening and closingtimes of the inlet valve 12 can be attained.

If the high-pressure pump 36 is not pumping, and in other words thehydraulic line 38 and the tie line 60 are pressureless, then the piston72 of the pressure reservoir 62 is in the position of repose shown inFIG. 2. In the graph of FIG. 3, in which the travel s of the piston 72of the pressure reservoir 62 is plotted over the hydraulic pressure p,this position of repose is at a position identified by reference numeral94.

If the high-pressure pump 36 is switched on, the pressure in thehydraulic line 38 and the tie line 60 rises. Since the cup springs 88have a lesser stiffness than the cup springs 90, the elongated tubularpart 80 initially remains stationary during this pressure increase,while conversely the piston 72 moves in the direction of the cap 68 ofthe housing 64 and in the process compresses the cup springs 88.

The spacing between the left-hand axial end, in terms of FIG. 2, of theelongated tubular part 80 and the piston 72 is selected such that thepiston 72 comes to rest on the elongated tubular part 80 whenever theminimum operating pressure PBMIN is reached. The corresponding travelaccomplished by the piston 72 is shown in FIG. 3 as SPBMIN. The geometryinside the pressure reservoir 62, and in particular the length of theleft-hand end portion 82 of the elongated tubular part 80, is selectedsuch that whenever the piston 72 comes to rest on the elongated tubularpart 80, the cup springs 88 have not yet moved into a block.

If the pressure is increased further, then the elongated tubular part 80is moved by the piston 72 in the direction of the bottom of the recess92 in the cap 68 of the housing 64. As a result, the cup springs 90 aredeformed. Since the cup springs 90 are considerably stiffer than the cupsprings 88, in this range a markedly greater slope of the curve shown inFIG. 3 results. The spacing between the right-hand axial end, in termsof FIG. 2, of the elongated tubular part 80 and the bottom of the recess92 in the cap 68 is selected such that whenever the hydraulic pressurereaches the maximum operating pressure PBMAX, the elongated tubular part80 comes to rest on the bottom of the recess 92 in the cap 68. Thelength of the right-hand end portion 84 of the elongated tubular part80, in turn, is selected such that whenever the elongated tubular part80 touches the cap 68, the springs 90 of the group 89 have not yet beencompletely deformed. The piston 72 in this case has covered the maximumpossible travel SPBMAX.

When the hydraulic system 10 is in its normal operating state, thehydraulic pressure in the hydraulic lines 38, 42, 46 and 60 is in therange between the minimum operating pressure PBMIN and the maximumoperating pressure PBMAX. In this case, the pressure reservoir 62functions as a vibration damper for pressure fluctuations that occur inthe hydraulic fluid of the hydraulic system 10. Because of the greatstiffness of the cup springs 90, even major amplitudes of the pressurevibrations cause only a slight motion of the piston 72. The length ofthe packet 89 of cup springs 90 can therefore be slight, which in turnreduces the total structural length of the pressure reservoir 62.

The great stiffness of the cup springs 90 also makes it possible toreduce the fluid volume stored in the pressure reservoir 62. This makesthe desired vibration damping in the operating pressure range possible,without impairment of the system dynamics of the hydraulic system 10.Moreover, the use of the cup springs 90 improves the damping action ofthe pressure reservoir 62, since major friction damping occurs betweenthe individual cup springs 90.

Compared to a conventional pressure reservoir, the pressure reservoir 62shown in FIG. 2 is very small in size. If vibration damping in the sameoperating pressure range is to be furnished in a conventional pressurereservoir, the conventional pressure reservoir would have to have amarkedly longer spring travel and thus a markedly greater structurallength. This is represented by dashed lines in FIG. 3. The spring travelrequired in a conventional pressure reservoir for the same operatingpressure range and the same emergency pressure properties is markedSPMAX′ in FIG. 3. The reduction in structural length for the pressurereservoir 62 compared with a conventional pressure reservoir thusamounts to the difference between SPMAX′ and SPMAX.

Upon a pressure drop inside the hydraulic system 10, for caused by afailure of the high-pressure pump 36, assurance must be provided thatthe piston 20 of the hydraulic cylinder 16 can still be moved far enoughupward that the inlet valve 12 can be closed. This is necessary toprevent the valve element 26 of the inlet valve 12, which elementprotrudes into the combustion chamber 30, from colliding with othervalve elements or even with the piston (not shown) in the combustionchamber 30.

In such a case, the cup springs 90 and especially the cup springs 88press the piston 72 in the pressure reservoir 62 back into its extremeleft-hand position in FIG. 2. Correspondingly, a hydraulic fluid volumeis forced out of the pressure reservoir 62 into the tie line 60 and fromthere via the branch line 42 into the lower work chamber 44 of thehydraulic cylinder 16. The spring travel of the cup springs 88 and theresultant movement distance SPMIN of the piston 72 is selected such thatsecure closure of the inlet valve 12 is possible in every situation.Thus in the normal operating range, a pressure reservoir 62 with optimaldamping properties is available, while conversely, in the event of apressure drop, the same pressure reservoir 62 furnishes a sufficienthydraulic fluid volume for secure closure of the inlet valve 12 via thehydraulic cylinder 16.

In FIGS. 4-7, further exemplary embodiments of pressure reservoirs 62are shown schematically. Elements whose function is equivalent to thoseshown in FIG. 2 are identified by the same reference numerals. They willnot be described again in detail.

In the exemplary embodiment shown in FIG. 4, an elongated tubular part80 is omitted. Instead, the springs 88 and 90, shown only symbolically,of different stiffness and different length are integrally joinedtogether.

In the exemplary embodiment shown in FIG. 5, instead of cup springs orhelical springs, air springs 88 and 90 are used, which have differentvolumes and different fill pressures.

In FIG. 6, springs of equal stiffness are used, but these are springsdisposed parallel, with different lengths. The spring 88 disposedcentrally in FIG. 6 has a greater length than the two springs 90disposed laterally of the spring 88. In this way, in a first range ofmotion of the piston 72, located adjacent to the position repose, onlythe spring 88 is initially acted upon, while conversely in a secondrange of motion of the piston 72, the springs 90 are acted upon as well,as a result of which the total spring stiffness increases.

In FIG. 7, instead of springs, an electromagnet 88 is used, which exertsa repellent force on the piston 72 made of a permanent magneticmaterial. The repellent force can be adjusted by means of a controller96 as a function of the position of the piston 72, which position isdetected by a sensor 98.

The foregoing relates to preferred exemplary embodiments of theinvention, it being understood that other variants and embodimentsthereof are possible within the spirit and scope of the invention, thelatter being defined by the appended claims.

We claim:
 1. A pressure reservoir (62) for exerting pressure on ahydraulic system (10), the pressure reservoir comprising, a housing (64,68) a piston (72), and prestressing means (88, 90), prestressing thepiston (72) of the pressure reservoir (62) during operation, theprestressing means having a characteristic force-travel curve, in onerange of motion of the piston (72), that has a slope which differs fromthe slope in a different range of motion of the piston (72), saidprestressing means (88, 90) having at least two series-connected devices(88, 90), which have characteristic force-travel curves of differentslope and which prestress the piston (72) in operation, said at leasttwo series-connected devices (88, 90) comprising at least twoseries-connected springs and where the stiffness of at least one spring(88) differs from that of at least one other spring (90), wherein thepressure reservoir (62) further comprises an elongated part (80) withtwo end portions (82, 84) and one support portion (86), which isdisposed between the end portions (82, 84) and has a larger outerdimension than the end portions (82, 84) and on which two adjacentsprings (88, 90) are braced, the at least one spring (88) beingtightened in operation between one side of the support portion (86) andthe piston (72), and the at least one other spring (90) being tightenedbetween the other side of the support portion (86) and a housing portion(68).
 2. The pressure reservoir (62) of claim 1 further comprising atleast two stops, which stops prevent the springs (88, 90) from beingtightened into a block in operation.
 3. The pressure reservoir of claim1 wherein the length of the elongated part (80) is adapted such that oneaxial end of the elongated part (80) forms a stop with a housing portion(68) of the pressure reservoir (62), and the other axial end of theelongated part (80) forms a stop with the piston (72).
 4. The pressureof claim 1 the length of the elongated part (80) is adapted such thatone axial end of the elongated part (80) forms a stop with a housingportion (68) of the pressure reservoir (62), and the other axial end ofthe elongated part (80) forms a stop with the piston (72).
 5. Thepressure reservoir of claim 1 wherein at least one of the springs (88,90) is a cup spring.
 6. The pressure reservoir of claim 3 wherein atleast one of the springs (88, 90) is a cup spring.
 7. A hydraulic system(10) for actuating a gas exchange valve (12) of an internal combustionengine (14), the system including a fluid reservoir (34), a fluid pump(36), a fluid line (38, 42, 44, 54, 60), a pressure reservoir (62) thatcommunicates with the fluid line (38, 42, 44, 54, 60) and has a housing(64, 68) and a piston (72) prestressed in operation by prestressingmeans (88, 90), and an actuating device (16), which communicates via avalve device (48, 56) with the fluid line (38, 42, 44, 54, 60) andactuates the gas exchange valve (12), the prestressing means (88, 90)having a characteristic force-travel curve, in one range of motion ofthe piston (72), that has a slope which differs from the slope in adifferent range of motion of the piston (72), and including at least twoseries-connected springs (88, 90), the stiffness of at least one spring(88) differing from that of at least one other spring (90), wherein thepressure reservoir (62) further comprises an elongated part (80) withtwo end portions (82, 84) and one support portion (86), which isdisposed between the end portions (82, 84) and has a larger outerdimension than the end portions (82, 84) and on which two adjacentsprings (88, 90) are braced, the at least one spring (88) beingtightened in operation between one side of the support portion (86) andthe piston (72), and the at least one other spring (90) being tightenedbetween the other side of the support portion (86) and a housing portion(68).
 8. The hydraulic system of claim 7, wherein said pressurereservoir further comprising at least two stops, which stops prevent thesprings (88, 90) from being tightened into a block in operation.
 9. Thehydraulic system of claim 7 wherein at least one of the springs (88, 90)is a cup spring.